Transmission control in signaling systems



Sept. 19, 1933.

N. D. NEWBY ET AL TRANSMISSION CONTROL IN SIGNALING SYSTEMS Filed March 1o, i951 3 Sheets-Sheet 1 A TTORNEV Sept. 19, 1933. N. D. NEWBY ET AL 1,927,133

TRANSMISSION CONTROL IN SIGNALINGl SYSTEMS Filed March l0, 1931 3 Sheets-Sheet 2 Ni D. NEWBV A TTORNEV Sept. 19, 1933. N. D. NEWBY ET AL TRANSMISSION CONTROL 1N SIGNALING sYsTEMs 5 Sheets-Sheet 3 @EQ .AQ 4 W VHN? Vv, b BQ :RS BQ MQ BD M JN db.l m TlT @ELM NS W zh. mm* ,Wl LST. l NM D.L. v\\ n v| M \\Q\ .D n H \vN D A l O h N V, n W 5 w s s maf-Ilm patented Sept. 19, M333 uirsosrirrss iszun TRANSMISSEON cominci. 1N SIGNALING'- sYsrsMs Neal l). Newby, Woodbridge, N. J., and Dwight L.

Moody, Hempstead,

N. Y., assignors to American Telephone and Telegraph Company,

a corporation of New Application March 10,

l1 Claims.

.This invention .relates to two-way signaling systems and particularly to two-way toll telephone systems equipped with voice-operated echo Suppressors, and utilizing auxiliary alternating current signals for operating calling. and supervisory apparatus at the ends of the signal transmission line.

The voice-operated echo Suppressors of the prior art impose fundamental limitations on voice frequency signaling over toll telephone lines. One of these limitations is that the Suppressors ordinarily'will operate to prevent duplex signaling over the toll line. Another is that they will prevent signaling out over the toil line while speech or tones are coming in the reverse direction. These limitations do not prevent satisfactory operation on most toll lines' in use at present in which the echo Suppressors are operated by the so--called ring-down method which allows the operators to keep on signaling until the `voice energy or tones corm'ng in the reverse direction are interrupted long enough to permit the signals to get through. use some sortvcf automatic control of signaling on these toll lines, such as would be used, for example, with voice frequency, multi-channel,

straightforward or dialing operation, echo suppressors of the present design could not be used unless more or less complicated and expensive auxiliary circuitsare utilized for relaying kthe signals around the Suppressors. v f

An object of the invention is to improve the operation of a two-way toll telephonev system equipped with voice operated echo suppressors and employing auxiliary alternating current signals for controlling thecalling and supervisory signal apparatus at the ends or" the system.

The invention isa two-way tolltelephone system in which the voice operated echo suppressor duplexI direct transmission or alternating current calling or supervisory vsignals over the system at all times, even while speech waves or tones are coming Vin the reverse direction.

This is accomplished in a preferred embodiment by relay means associated with each echo suppressor and responsive to received calling and supervisory signals to render the suppressors ineffective thus allowing the signals to be freely transmitted over the toll line in either direction, while allowing the Suppressors to function in the usual manner to suppress echoes or to prevent singing in response to transmitted speech waves or other signals when the calling or supervisory signals are not being transmitted.

However, if it is desired tov Vcircuits are arranged so as to rallow full or hali` York 1931. Serial No. 52L586 (Cl. liti- 170) Y The exact nature of the invention and its advantages will be understood from thenfollowing detailed description thereof when read in connection with the accompanying drawings in which:

Figure 1 `shows the invention applied to la fourwire telephone circuit employing echo Suppressors of the mechanical relay type; y

Fig. 2 shows the invention applied to a twowire telephone circuit employing echo Suppressors of the mechanical relay type; and v Figjshows the invention applied to a fourwire telephone repeater circuit equipped with echo Suppressors of the vacuum tube type.

Fig. l shows the invention applied to a four- Wire telephone circuit employing echo suppressors. The telephone circuit comprises a one-way, two-wire path EA including the one-way amplifying devices 1, v2 and Sadapted for repeating in amplified form telephone currents received from a west vrtwo-way telephone lineY section L] to an eastV two-way telephone line section L2,and a one-way repeating path WA including the oneway amplifiers Li, 5 and 6 adapted for repeating 'in ,amplied form telephone currents vreceived from the east two-way line section L2 to the west two-way line section L1; The input of the repeating path EA and the output of the repeating path WA are connected in energy transmitting relation with the west line ksection L1 and-in" substantially conjugate relation with 'each other by means of ,thev three-winding transiornieror hybrid coil H1 and associated balancing network N1, and the output ofthe repeating path EA and the input ofthe repeating path WA are con-- nected in energy transmittingrelation ywith the east line section L2, and in substantially conjugate relation with each other by means of the threewinding transformer'or hybrid coil H2 and associated balancing network N2, inwell known man.

ner.

path WA `in the absence oi signal transmission thereover are normally operative. l f

The normally deenergized mechanical relay '7 is adapted when energizing current is supplied to its winding to open the normally closed contacts 8, 8 in the repeating path EA to disconnect the east portion of the path EAv from the west portion thereof, to close normally open contacts 9, 9 to connect the short-circuiting connection 10 acrosstheeast portion of the path EA, and to'V close normally opencontacts 1l, l1v to connect a suitably designed balancing network 12 to the west portion of the amplifying path EA.

Both. the-repeating path and the repeating Similarly, adjacent the amplifying path WA is a normally deenergized mechanical relay 13 adapted, when energizing current is supplied to its winding, to open the normally closed contacts 14, 14 in the path WA, to disconnect the east portion of the path WA from the west portion thereof, to close normally open contacts 15, 15 to connect the short-circuiting connection 16 across the west portion of the path WA, and to close normally open switches 17, 17 to connect al suitably designed balancing network 18 across the east portion of the amplifying path WA.

Also, the relay 7 when energized opens the normally closed contact 19 in the energizing circuit for relay 13, and the relay 13 when energized opens the normally closed contact 20 in the energizing circuit for relay 7.

. Connected across the one-way amplifying path EA between the point of connection therein of the normally closed contacts 8, 8 and the hybrid coil H1 and preferably in the output of an amplifier in that portion of the path EA is the input` of the suppressor controlling transmission in the path WA, which will be referred to hereinafter as the west suppressor. It comprises the Vacuum tube circuit 2i, the input of which is connected across the path EA, and the chain of relays shown within the dot-dash box 22 in the output of the circuit 21 and controlled thereby. Connected across the one-way amplifying path WA between the point of connection of the normally closed contacts 14, 14 therein and the hyv brid coil H2, and preferably in the output of an ampli-er inA that portion of the path WA, is the input of the suppressor controlling transmission in the path EA, which will be referred to hereinafter as the east suppressor. It comprises the Vacuum tube circuit Y2l', the input of which is connected across the path WA, and the chain of mechanical relays shown within the dot-dash box 22', in the output of the circuit 21 and controlled thereby. y

The chain of mechanical relays within the dotdash box 22 controls the operation of the rnechanical relay 13 adjacent the amplifying path WA and the energizing circuit for relay 7, and the chain of relays within the box 22 controls the operation of theY relay 7 adjacent the path EA and the energizing circuit for the relay 13, in the manner which will be described below. l1he Vacuum tube circuit 21 and the relay chain 22' in the east suppressor is similar to the Vacuum circuit 21 and the relay chain 22, respectively, in the west suppressor and operates in similar manner. Only the latter, therefore, will be described in detail. 'En the following description the apparatus in the circuit 21 and in the box 22 in the east suppressor will be identined by the same characters as the corresponding apparatusv in the west suppressor, but followed by a primeV mark. Y

The input of the vacuum tube circuit 21 is designed to offer a high impedance to the portion of the repeatingpath EA to which it is connected so that it will not appreciably affect the direct transmission of telephone waves over the path EA. The power taken from the path EA by the associated echo suppressor istherefore relatively small, requiring amplication in order that it may control the operation of the relay chain within the dot-dash box 22. The vacuum tube circuit 21 comprises the input transformer 23 and the two three-electrode vacuum tubes 24 and 25 connected in tandem through the interstage transformer 26. The input transformer 23 is broadly tuned by the condenser 27 in shunt to its secondary winding-and by the capacity introduced by the transformer windings and the input capacity of the tube 24 so as to transmit efficiently to the tube 24 incoming currents Within the voice frequency signaling range, but to be inefficient for transmitting other frequencies, particularly those below 500 cycles per second. Some protection is thereby secured against operation-of the tube 24 by low frequency noise currents received from the path EA. Input transformer 23 has taps as shown by which the Voltage applied to the grid of the tube 24 and consequently' the sensitivity of the vacuum tube circuit 21 can be readily adjusted.

The three-electrode vacuum tube 24 is arranged to function as an amplifier, and the tube 25 is arranged to function as a combined amplifier and C bias detector the output current of which controls the operation of the relays shown Within the box 22. The coupling circuit between Ythe tubes 24 and 25 and including the interstage transformer 26 is designed to provide maximum transmission efficiency in the range of important voice'frequencies. The grid of the detector tube 25 is supplied by the grid battery 28 with a relatively large negative potential so as to reduce the space current of the tube to zero, or nearly so, when no alternating current voltage is supplied to the input electrodes of the tube 25 through the interstage transformer 26. When voice currents are being transmitted over the repeating path EAY in the direction from west to east, a

portion thereof will be impressed by the input transformer 23 of the vacuum tube circuit 21 on the input of the amplifying tube 24 and amplified thereby. The amplified Voice waves in the output of the amplifier 24 will be impressed by the interstage transformer 26 upon the input elecf trodes of thek detector 25. The instantaneous values of the alternating voltage impressed upon the grid of the detector 25 which make the grid Amore negative produce no effecten the output current of that tube, but instantaneous values of the impressed voltage which makes the grid of the tube 25 more positive cause pulses of current to be produced in the plate circuit of the detector tube. The small condenser 29 shunted across the output of the detector tube 25 `dash box 22, energizing that winding. The energization of the winding of relay 30 causes that relay to operate to close the normally open contacts 31 to close an energizing circuit from the source of direct current 32 through one operating winding 33a of the relay 33, and to close an energizing circuit from the source of direct current 34 through the operating winding 35a of the hang-over relay 35. Relay 33 will then operate to close the normally open contacts 36 to close an energizing circuit from battery 37 'to the winding of relay 13. Relay 13 will then operate to open the normally closed contacts 14, 14 in.

the repeating path WA, disconnecting the west portion of that path from the east portion thereof, to close normally open contacts 15, l5 to connect the short-circuiting connection 16 across the west portion of the path WA, and to close 0 gio signals, the signaling frequencies which are used to interrupt the carrier should be preferably chosen outside the syllabic frequency range.

Although the suppressor is not functioning as such when interrupted signaling currents are passing over both sides of the circuit simultaneously, if the double carrier, duplex signaling system is used, (different carrier frequencies in opposite directions), the echo must pass over the circuit twice before reaching a ringer which it may affect, that is, a ringing signal which is sent from a west station associated with the line section L1 passes over the repeating path EA and line section L2 to an east station associated with the line section L2 where it actuates a ringer. The echo of that ringing signal passes back over the repeating path WA to the west station. The ringer at the west station is adjusted for a different ringing frequency, that of the ringing signal originating at the east station, and is, therefore, not affected by the echo. The echo may be again reflected at the west station and passed over the repeating path EA to the east station again. lt should be noted that since this echo is passed over the circuit twice its level will be materially lower than that of the ringing signal itself and should not, therefore, affectthe ringer at the east station.

Fig. 2 shows a modified form of the invention applied to a two-wire repeater circuit equipped with echo Suppressors of the mechanical relay type. The repeater circuit comprises a normally operative one-way path EA including the oneway amplifying device for repeating in amplined form telephonie currents in the direction from west to east between a west two-way line section L1 and an east two-way line section L2, and a one-way amplifying path WA including the one-way ampliner 5l for repeating in amplified form telephonie currents in the direction from east to west between the east line section L2 and the west line section L1. The input of the amplifying path EAand the output of the amplifying path WA are connected in energy transmitting relation with the two-way line section L1, and in substantially conjugate relation with each other by means of the hybrid coil H1 and associated balancing network N1, and the output of the repeating path EA and the input of the repeating path WA are associated in energy transmitting relation with the line section L2, and in substantially conjugate relation with each other by means of the hybrid coil H2 and associated balancing network N2, in well known manner.

Connected across the repeating path EA at the points 52 in the input of the amplifier 50 therein is the input of the west echo suppressor comprising the vacuum tube circuit 53 and the chain of mechanical relays controlled thereby shown within the dot-dash box 54. Connected across the repeating path WA at the points 52' in the input of the amplifier 5i therein is the input of east echo suppressor comprising the vacuum tube circuit 53 and the chain of mechanical relays controlled thereby shown within the dotdash box 54.

As the elements in the east suppressor are identical with those in the west suppressor and similarly arranged, it will be only necessary to describe in detail those in the former. In the following description apparatus in the east echo suppressor corresponding to that in the west echo suppressor will be identified by the same characters, but with a prime mark following.

The vacuum tube circuit 53 in the west echo suppressor is connected across the repeating patli'. EA at the points 52 through the input transformer 55 which is arranged to provide a high voltage step-up. The resistances 56 and 57 of suitable value are connected in series with the primary winding of the input transformer 55 for the purpose of reducing the bridging loss of the input impedance of the suppressor on direct transmission through the repeating path EA. The Vacuum tube circuit 53 comprises the three-electrode vacuum tube 58 operating to amplify the voice energy taken from the input of the repeating path EA, and the three-electrode vacuum tube 59 acting as a combined amplifier and detector to convert the amplied Voice energy into pulsating unidirectional current suitable for operating relays. The Vacuum tubes 58 and 59 are connected in tandemr through the interstage transformer 60 which is designed to give a high voltage step-up for transmitted waves of frequencies in the neighborhood of 1000 cycles per second so that the vacuum tube circuit as a whole provides maximum transmission efciency at this frequency and somewhat less efficiency at other frequencies, particularly below 550 cycles per second. This provides some protection from low frequency noise currents, and makes the vacuum tube circuit more sensitive at frequencies where the power in the direct transmission is strong. v Y

The vacuum tubes 5S and 59 in the vacuum tube circuit 53 have the usual filament heating,

grid biasing and space current supply batteries..

Negative grid potential for the amplifier tube 58 is obtained from a porti-on of the voltage drop in the filament circuit of the tubes. The circuit is designed so that a sufficient negative biasing potential is applied to the grid of the detector tube 59 to insure that no plate current will flow throughthe relays in the plate circuitV when no voltage is applied to the input of the vacuum tube circuit 53 from the repeating path EA.

In the relay chain controlled by the vacuum tube circuit 53 and shown within the dot-dash box 54 is a polarized relay 6l, which will be referred to hereafter as the detector relay as it is connected in the plate circuit of the detector 59 so as to be operated directly by impulses therefrom, and a polarized relay 62 acting as a hangover relay. rhe detector relays 6l and 6l' of the two Suppressors are connected in a differential arrangement. One winding 61a of the detector relay 6l is connected to the corresponding winding 61a/ of the detector relay 6l in the other suppressor in such manner that when a current impulse is produced in the plate circuit of the detector tube of one suppressor, for example, in the plate circuit of the detector tube 59 in the west echo suppressor, causing the winding 61a of the detector relay 6l to be energized, the same energizing current flows also through the winding 61d of the detector relay 61 in the east echo suppressor in such direction as to put strong bias on that relay, tending to prevent it from being falsely operated by reflected current due to irregularities in the repeating paths.v

Plate battery of the detector tubes 59, 59 is fed through the two detector relays as shown, and between the two windings of theserelays a resistance 63 to ground is placed. Plate current is supplied to the plate of the detector tubes 59', from the plate battery 6e through retard coil 65, the windings 61h, 61h of the detector relays and resistance 63 thus biasing relay 61 so that when the Suppressors are in the non-operated condition, the armature of the relay is held detector relay 61 to release and its armature` Will tall back from its front Contact 72 to its. back` contact 66 to short-circuit the energizing.

against the yback contacts 66. 1n a similar manner plate current is supplied to the plate of detector tube 59 from ,batteryy 75 through retard. coil 76 and windings 61d and 61u',y and resistance.

63 thus properly biasing relay 6l. v

The negative grid potential applied to each detector tube 5, 59 reducesits space current to Zero or nearly so when no voltages are being impressed on the input oi the vacuum tube circuits 53, 61, 6l are normally in a released condition with the armatures held to their back contacts 66, 66'-by biasing current from batteries 64; and 7 When voice currents are being transmitted over the repeating path EA a portion is irnpressedcn the input of the vacuum tube circuit .53 of the West suppressor through the input t) -sforiner 55. The impressed voice currents are amplifiedv by the vacuum tube amplifier 56 and the amplied alternating voltages are impressed upon the grid of the detector tube 59 through the interstage transformer 60.v Instantaneous values of this voltage Which make the grid oi tube 59 more negative, produce no eiiect on the space current of the tube, but instantaneous values of the irnpressed voltage which maire the grid more positive cause corresponding pulses or" current to appear in the plate circuit of tube 59 and pass through the Winding 61u of thepdetector relay `.l causing that relay to operate and the Winding 61a or detector relay 6i biasing the latter relay against operation. The condenser 67 is utilized to by-pass the alternating current coinponent of vthe rectiiied current inthe plate cir- "cuit of the detector tube 59 from the detector relays so as to prevent chatter.

Now, if the alternative voltage due to speech transmission in path EA, applied to the input of vacuum tube circuit 53 is of sufficient magnitude, the resultant plate current in the detector tube 59is of sufficient value to cause the operation of the detector/relay 61 and to cause a strong bias to be placed on the correonding Winding ci the detector relay 61' in The armature of re- As soon echo suppressor. lay V6l will leave its back contact r66.

as the armature of relay 6l has moved from the Contact 66, short-circuiting ground is removed` circuiting connection 71 across the repeating path WA in the input of the ainpliner 51 there by preventing transmission of speech. currents over that path until the short circuit has been moved. Meanwhile, the armature of the de- Lector relay 61 will have moved from the back Contact 66 to the front contact 72 discharging the condenser 72 through a now from the grounded battery 75 through retardation coii 76, the Winding 62o of suppl. or ceases. the current in the de lector tube 59 will be reduced to zero, causing Consequently detector relaysl resistance 74 to ground, and at the saine time Will cause currentl The current flowing through thecircuit for winding 62a of hang-over relay 62 reducing the current'therein'to zero by grounding the battery '66 through resistance 71, and removingthe lground frorrL Winding 62h of the hang-over reiay However, the armature of the hang-over relay 62 will not iirnrnedately iai since current from. the battery 75 Will nue to iiow t rough the winding 62h of relay and resistance 77 until the condenser depends upon the value of capacity o1.' the condenser cnd is time interval, the hangivill release moving its armature p i to lcontact 69 and thus producing an open circuit in the connection 71 short- ...cuil-,ing the path WA. The path WA is now operative so as to transmit speech Waves thereover from the line section L2 to theiine section L1 in the direction from eastto west. Current At the from battery'l 90 through resistance 91 flows through windings 62o and 62e of relays 62 and 62 to bias these relays so that the armature normally closes contacts 69Y and 69.

So far, the resistance 78 in series inone line V Wire of the path EA in the input of amplifier 50 and the corresponding resistance 78 in the input of aznplier 5i in repeating' path V/A have not been mentioned. riheir function is to prevent the input of an echo suppressor fromv being short-circuited Whenv the associated re-` peating path is short-circuited.y The resistance 78 is normally short-circuited through the front contact 79 ofthe normally energized, slow-release relay 86 in the West suppressor and the back contact 69 of the hang-over relay 62 in the east suppressor so that its presence does not affect transmission over the repeating path EA. Similarly, the resistance 78 is normally shortcircuited through the front contact 79 of slowrelease relay 80 in the east suppressor and the back contact 69 or" hang-over relay 62 in the West suppressor so thatl its presence does not ailect transmission over the repeating path WA.

When the( repeating path WA is short-circuitedr by operation of. the West echo suppressor in the manner described above, the short-circuiting rents in that path received from the line section L2 flow through the resistance 78 produce a Voltage drop in the resistance which is proportional to the speech voltage applied to the input of the West suppressor. Resistance 78 'in repeatingpath EA in similar manner when speech currents are received from the line section L1, serves to apply the speech currents to the West echo suppressor when the associated repeating path is. short-circuited by operation of the east echo suppressor.

Relays 86, 81. and 82 and their associated leads, in the west echo suppressor, a--d the corresponding. relays 80H81 and'82 and their associated leads in the east echo suppressor, provide means fully charged. The length of time that.

llii.

for rendering the Suppressors ineiT-ective as long `ais-calling or supervisory alternating current signals of certain characteristics such as a voicerequen'cy carrier interrupted at the proper signalingrate, say 20, 30 or l5 times `per second,"

is impressed on either side of the suppressor. The frequency of interruption should be Well above the fr ncyitpossihle. The conditions under which relays 80, 81 and 82 operate and the manner of their operation are similar to that described above for the corresponding relays 40, 45 and 47 in connection with the system of Fig. 1.

A portion of the interrupted voice-frequency carrier transmitted over the repeating path EA is impressed upon the input of the vacuum tube circuit 53 and is amplified and detected yby the two tubes 58 and 59. rhe interrupted frequency of the carrier, therefore, appears in the plate circuit of the detector tube 59 and causes the operation of polarized relay 81 and polarized relay 82 connected thereto. The condenser 84 in series with polar relay 81 tunes the circuit in which it is connected broadly enough to receive the interrupting frequencies of the carrier. The operation of relay 81 causes condenser 85 to be charged and discharged through the windings of relay 82 by current from the battery 86 at the signaling frequency. Relay 82 is thereby operated and causes the release of the slow-release relay 80. The slow release of this relay may be necessary in order to guard against false operation thereof in responsev to voice current impressed on circuit 53. The relay chain consisting of relays v82 and 81 is Varranged to operate only on the interrupting irequency. If, at the time relay releases, no signals or voice currents are being impressed on the east echo suppressor from the repeating path WA, the only change in the circuit resulting is that the resistance 78 in the path EA is short-circuited through the baolf. contact of relay 80 instead of through the make contact 79 thereof and the back contact 69 of relay 62.

1f the repeating path EA has been previously short-circuited by the `operation of relay 62 in response to transmission of speech currents in the repeating path WA, the release of relay 80 removes the short circuit on the repeating path EA by opening its make contact 79, and short-circuits resistance '78 by the closing of the back Contact of relay 80. The repeating path EA is thereby opened to allow the calling or supervisory signals to pass from line section L1 to line section L2 regardless of Whether or not speech or signal currents are flowing in path WA. It is apparent that this action can take place in both sides of the echo suppressor simultaneously and that duplex signaling is, therefore, possible.

Fig. 3 shows the invention applied to a fourwire repeater circuit equipped with signal controlled echo Suppressors of the vacuum tune type. The repeating circuit comprises the one-way amplifying path EA adapted for repeating in ampliiied forni in the direction from west to east, signals received from the two-way line section Li to the two-way line section L2, andthe one-way amplifying path WA adapted for repeating in amplified form in the direction from east to west sigreceived from the two-way iine section Li to the line section Li. The input of the repeating path EA and the output of the repeating path WA are connected Vin energy transmitting relation with the two-way section L1, and in substantiallyconjugate relation with `each other by means of the three-winding .transformer or hybrid coil Hiand the associated balancing network N1. `Similarly, `the output of the path EA and the. input of the WA connected in energy transmit- .ing relation with the two-way line section L2, and in .substantially conjugate relation withleach other by means of the 'three-winding transformer or hybrid .coil E12-.and associated balancing network N2, in well known manner.

The repeating path EA comprises the one-Way amplifier l0 in the input of the path, the one-way vacuum tube amplifier 101 near the center of the path and the one-way amplifier 102 near the output of the path. The vacuum tube amplifier 101 vin repeating path EA comprises the input transformer 1,03, the three-electrode vacuum tube amplifying devices 104 and 105 connected in tandem through. interstage transformer 106, and output transform 107. The vacuum tubes 104 and 105 have the usual lament heating, grid biasing and space current supply batteries. The grid electrode of the first amplifying tube 10A in the am- `pliiier 101 receives a normal negative bias from the grid biasing battery 108 of such value as to make the tube 104 normally operative, to repeat signals impressed upon its input by the input transformer 103. The vacuum tube amplifying device 105 in the second stage of the amplifier 101 is also normally biased so as to be -operative to repeat signal waves impressed upon its input by the interstage transformer 106.

The apparatus in the repeating path WA is identical with that in the repeating path EA. The apparatus in the path WA cor-responding Ito that in the repeating path EA is designated by similar characters followed by a prime mark.

Connected across the repeating path EA in the output of the amplifier 101 therein is the input of the west echo suppressor comprising the input transformer 109, the three-electrode vacuum tube amplifier 110 and the three-electrode vacuum tube rectifier 111 having its grid and plate electrodes strapped together, coupled in tandem by the interstage transformer 112, and the filter 113 comprising the retardation coil 114, the resistance 115 and the condenser `116, connected between the output of the rectifier 111 and the grid circuit of the vacuum tube 104 in the vacuum tube amplifier 101 in `the repeating path WA.

Connected across the output of the .amplifier 101' in repeating path WA is the input `of the east suppressor comprising the same elements as specified above for the east suppressor, the elements therein as stated above being identified with the same characters as the corresponding elements in the west echo suppressor, except that they are followed by a prime mark. VIn the east echo suppressor, the filter 113 is connected between the output of the vacuum tube rectifier 111 and the grid of .the vacuum tube 104i in the ice first stage of the amplier 101 in the repeating are amplified by the normally operative one-way amplifiers and 101 therein. The main portion of the amplified speech waves in the 4output of the amplifier 101 are transmitted to the input of the one-way amplifying device 102 and are amplified thereby and the amplified waves impressed by the hybrid coil H2 upon the twoway line section L2. A portion of the speech waves in the output of the amplifier 101 is impressed upon the input of the west echo suppressor by the input transformer 109. The resistances and condensers in series with the primary winding of the input transformer 109 are provided for the purpose of reducing the bridging loss produced in the repeating path EA by the transformer 109. The speech waves impressed on the echo suppressor ,circuit by input trans-k former 109 are ampiied by the amplier 110 and are then irnpressed upon the input circuit oi the rectier 111 by interstagc transformer 112 and are rectified thereby. The grid and plate of the rectiiier tube 111, which are strapped together, are normally biased by current irorn the battery 117 so that normally no current flows through the rectifier tube 111.

When the speech waves are aniplied by the ampliiier and impressed on the rectii ing circuit of tube 111, the positive haii oi the waves causes current to ilow through the circuit which may be traced as follows. The circuit passes from ground through the iilainent oi tube 111, the electronic path between the iilainent and the plate and grid tied together oi the tube 111, the secondary winding of the interstage transformer 112, battery 117, retardation coil r11/1, resistance to the iilaroent tube 101i of the ainplier 101 in repeating path WA to ground. This pulsating current is ltered by the retardation coil 114 and the condenser 116 so that practically fteady D. C. c llows through resistance 115. The voltage drop caused by this current flowing through resistance 115 gives the grid of the repeater tube 10e towhich the resistance 115 is' connected, a further negative bias. The negative bias on the grid of tube 1011 in repeater 161 in the repeating path WA is now great enough to prevent operation ci that tube in response to speech currents impressed thereon through input transformer 103. .The tube les will remain blocked and therefore the repeating path 'WA will remain inoperative to transmit speech currents from two-Way line section L2 to the two-way line section L1 long as the speech waves trans mitted over repeating path EA maintain the west ocho suppressor operatedto supply the additional negative potential to the grid or" tube 164. 'When the input of speech waves from the repeating path EA to the west suppressor ceases, the rectiiied current passing through the resistance 115 will cease to iiow so as to reduce the negative po- The west echo suppressor associated with the output oi the repeater 101 in the repeating path WA operates in a siiniiar manner to that oi' the east echo suppressor just described in response to speech` waves impressed thereon to apply a negative bias to the'grid oi tube 104 in the ampliiier 101 in repeating path EA blocking that tube and thus preventing subsequent transmission lof speech currents over the path EA between the line sctions L1 and L2.

To render the echo suppressors ineilectiveto block the paths when calling or supern visory signals, as a voice frequency carrier interrupted at proper signaling rate, say, 2i), 3G or li5 times per second, is to be transmitted etween the two-Way line sections L1 and Lz, the

circuits connected across the input or" the ain.

from line section L1 to line section L2. These currents7 as well as any` speech currents which may be on the repeating path EA at the time, are impressed upon the threeeelectrode amplifying tube 118 by the input transformer 119. IThe resistances and condensers in series with the primary winding of the input transformer 112 are provided to reduce the bridged loss across the path iA'caused by the input After amplication bythe -anipli the interrupted signals, or the interru te and speech currents, are impressed oy th terstage transformer 12@ on the grid of the threeelectroole vacuum tube combined amplifier and Vacuum tube detector 121 the grid of which has a high negative bias provided by the grid battery 122 so that normally no current hows the plate circuit of the rectier tube 121. However, each positive portion of the wave upon the input circuit of the rectifier 121 through interstage transformer causes a pulse of current to flow in the plate circuit of the rectifier tube 121. The voice-frequency co'iponent o1 this current is shunted to the of tube 121 by condenser 123, leaving the interrupting fre duency oi the carrier in the plate circ This current of frequency say 20, Eil or l5 cycles per second flows through and causes the operation of the polarized relay 12/1, the condenser 125 in. series therewith tuning the circuit broadly enough to receive the interrupting freduencyoi the carrier.

The periodic operation of polarized'rele-.y 124 causes its armature to continuously maire and break connection with the contacts 125 causing the condenser 12'? to be alternatelycharged with current from battery 128 through. the windings relay 129 and discharged through those windings at the signaling frequency. Relay 129 is thereby operated to open the normally closed contact 13G causing the release oi slow-release relay 131 nore nially energized by current from battery 122. rhe release of relay 131 causes its armature to slowly fall back and finally make contact with contact 133 thereby closing an energizing circuit for the Winding of relay 13/1 from battery The slown release time of relay 131 inay be by any well known means suiiicient to pre-rent closing or the Contact 133 in response to voice currents. Relay 131 then operates to close contact 136 thereby shcrt-circuiting the secondary winding of' stage transformer 112 in the east echo supressors. The short-circuiting of the secondary winding oi interstage transformer 112 will prevent subsequent operation oi the rectifier 111 respense to speech waves received frein the repeating path WA, and there'i re will prevent the application of any additional grid bias to the grid of tube 1G11 in the ainpliner lill in repeating path EA to block that anipliiier. Therefore, Whether or not vcice-frequency currents are being impressed upon the east echo suppressor, the tube 104 will remain operative thus allowing the interrupted signals to be amplified by the ainm plier 101 and transmitted over the output of that path to thefline section L, interm rupted at the proper rate transmitted over repeating path WA from line section L1 will similarly cause operation of amplifier 118' and rectier 121 to control the chain of relays 1221', 129', 131 and 134i to disable the east echo suppresser connected to the output of aniplnier 101 in the repeating path EA so as to prevent the disabling of the amplifier tube 10i in the amplin iler 101' in the repeating path WA, so as to perinit lis.

signals interrupted at the proper rate to be transmitted over that path between line sections L2 and Li.

The circuit shown in Fig. 3 has been described for the case where the interrupting and. calling signals to be transmitted over the repeating path comprise voice-frequency carrier interrupted at a certain rate. The circuit described, however, will operate satisfactorily ii the calling orsuper'- visory signals to be transmitted over the repeating path are steady frequencies, ii the chain oi relays operating on the interrupting frequencies re replaced with a sharply tuned circuit to receive the signal frequencies which will cause the operation of apparatus ilar to that shown to disable the echo suppressor.

Many modifications in the circuits which have been illustrated and described may be made by persons skilled in the art without departing from the spirit and scope of the invention as dened by the appended claims.

One modiiication may consist in placing in the amplifier-detector circuit of the echo suppressorl sharply tuned iilters to block the band or bands 3f frequencies used for supervisory signals, say 9504059 cycles, so that the echo suppressor will not be caused to function when these frequencies alone are impressed on the line. A second arnpliherdetector circuit operating on these freiiuencies alone would be connected to the disabling relay chain. This arrangement would allow supervisory signals to pass over one side ci the line, say the EA branch, and speech currents to pass over the other side WA simultaneously. The above result inay also be obtained by having the disabling relay means not only disable the control means on the opposite path but also the control means on the same side. This modification may be made in the system ci Fig. l by causng ground to be supplied to the armature ci relay 30 from the make contact of relay 4'? as indicated by the dotted line between the ground connection to the armature or" relay 30 and the maire contact of relay 47.

What is claimed is:

l. A twc-way signal transmission system comprising stations, two 'normally operative one-way paths for respectively transmitting signals in opposite directions between said stations, control means connected to each one-way path and responsive tol transmission of signals of certain characteristics therein to disable effectively the other one-way path, and means responsive only to transmission oi signals of other characteristics in one of said one-way paths to render the control means connected to the other one-way path in effective to disable said one-way path.

2. The system of claim i and in which the lastmentioned means cor- .'rises means controlled by the control means connected to said one cne-way path when the of said other characteristics are impressed thereon from said one one-way path, to break effectively the circuit oi the control means coni ected to said other one-way path.

3. The system of claim l in which the signals oi said certain and said other characteristics differ on a fred-uc' cy basis, and in which the lastmentioned means comprises a circuit comprising :i chain of mechanical relays responsive to operation oi the control means connected to said one one-way path when signals of said other characteristics impressed thereon, to break effectively the circuit ci the control connected *o the other one-way path, said relays having 'such operate and release times asvto malte the relay circuit unresponsive to operation of the control means by the frequencies of signals of said certain characteristics.

The system of claim l in which the signals eration oi the control means by said lower ire-j quency signals.

5.111 a two-way signaling system, oneeway paths for the transmission of signals between stations in opposite directions, wave-controlled means connected to each path and responsive to signal transmission thereover for effectively disabling the other one-way path so as to prevent transmission thereover between said stations, and means controlled by tl e wave-controlled means connected to each path only when signal waves ci cert in characteristics are impressed thereon to render the wave-centrolled means connected to the oppositely directed transmission path ineffective to prevent free transmission of said signal waves of said certain characteristics between said stations over the rst-mentioned path, while allowing the operation oi the wave- `controlled means connected to said oppositely directed path in response to signals of other characteristics transmitted therecver to disable said first-mentioned path when signals ci said certain characteristics are not being transmitted over said rst-mentioned path.

6. A two-way telephone transmission system comprising stations, a source of telephonie waves and a source of other signal waves of frequencies within the telephoni frequency range, interrupted at a certain rate, at each station, two one-way paths for respectively transmitting said telephonie and other signal waves in opposite directions between said stations, a control circuit connected to each path and responsive to transmission of waves within the telephonie frequency range over that path for effectively disabling the oppositely directed path, means connected to each control circuit and operative only when said interrupted signals are impressed thereon from the associated Vone-way path for rendering the control circuit connected to the other path ineffective thus permitting direct transmission of said interrupted signal waves between said stations over said one-way paths at all times.

7. The system or claim l and in which said means responsive to transmission ci signals ofV means connected to said one one-Way path are independent.

9. In a two-way signal transmission system, one-way paths including amplifying means for respectively repeating signals between stations in opposite directions, wave-controlled means connected to each one-way path and responsive to transmission thereover of signals within the voice frequency range for effectively disabling the oppositely directed one-way path, said means comprising relay means adapted' when operated to short-circuit said oppositely directed path and a vacuum tube amplifier-rectifier circuit having its input connected across said one one-way path and responsive to said signals within the voice frequency range to operate said relay means, and means controlled by the vacuum tube amplifier-rectifier circuit connected to one path in response only yto impressed signals outside the voice frequency range for preventing the relay means controlled by the vacuum tube amplifier-rectifier circuit connected to the other one-way path from short-circuiting the first one-Way path.v p

10. In a two-way signaling system, two normally operative one-Way transmission paths each including a space discharge amplifying device including a grid electrode, for respectively repeating signals in opposite directions between stations, control means connected to the output of the space discharge amplifying device in each one-way path and responsive to amplified signals of certain characteristics therein, for so biasing the grid electrode of the space discharge device in the other one-way path so as to block transmission therein, and means connected to each one-Way path in the input of the space discharge device therein and responsive only to signals of other characteristics in said one-way path to tube circuit, to disable the control means con- 1 nected to the other one-way path, in its output.

NEAL D. NEWBY. DWIGHT L. MOODY. 

